Metal strips whose thickness has been reduced to less than 5 mm by multiple rolling are not flat. They are wavy and show bulges. For this reason they are frequently unsuitable for further processing. Such strips are therefore subjected to a levelling process. In levelling, a distinction is drawn between tensile stretching and stretching-and-rolling levelling (sometimes referred to herein as stretching-and-bending levelling or tensile rolling). In tensile stretching the strip is subjected to tensile stressing in which its yield point stressing is exceeded, so that the strip becomes plastically deformed, namely lengthened. In stretching-and-rolling levelling the strip is subjected to tensile stressing and bent several times under tension in opposite directions. In that case also the yield point stressing is exceeded in the zone of the bending in alternate directions, the strip becoming permanently lengthened. Lastly, it is also known to combine tensile stretching and stretching-and-rolling levelling with one another, stretching-and-rolling levelling being performed first, followed by tensile stretching. In any case, in that prior art tensile stressing is considered necessary if stretching-and-rolling levelling is performed with a degree of stretching &gt;1%, since then allegedly due to residual stresses in the strip bulging occurs in the strip and must be eliminated by the following purely tensile stretching in the plastic zone (DE 35 25 343 C1). In practice the various levelling processes, whether purely tensile stretching, or stretching-and-rolling levelling, or combined stretching-and-bending levelling and tensile stretching, are unsatisfactory not only because the strip lacks satisfactory flatness after the treatment, but more particularly because the sheet again loses its flatness after a prolonged storage period.
There is also the aspect that in the case of purely tensile stretching and also in combined stretching levelling and tensile stretching no clearly-defined strip stretching can be ensured over a strip width over the purely tensile distance. Since the strip is placed under tension between two tensioning drums, it cannot be readily stated in which zone of the tensioning distance the strip stretching (lengthening) takes place. If the tensioning distance is defined as the distance between the centre points of the two tensioning rollers, the required strip lengthening will not take place in said zone, but it must be assumed that the stretching zone lies immediately before and after the vertex of the two tensioning rollers respectively. The required strip lengthening therefore takes place on the contact arc of the tensioning rollers, namely immediately before a run-off point of the decelerating roller or immediately after the run-on point of the pulling roller, because the stress introduced into the strip at those places is higher than over the actual tensioning distance, since at that place a bending stress depending on the roller diameter is superimposed on--i.e., added to the tensile stress generated by the tensioning rollers.
The lengthening of the strip on the periphery of tensioning rollers causes considerable disadvantages. Since in practice such tensioning rollers always have a rubber coating, errors in the shaping of the rubberizing, different layer thicknesses and uneven wear on the coating and changing rubberizing properties produce in the strip a pattern of stresses which does not correspond to the pattern of internal stresses of the strip to be levelled. The result is a strip which due to these many imponderables has definite residual stresses and therefore inadequate flatness.
It is an object of the invention to develop a process which obviates the aforementioned disadvantages and by means of which not only can a flat strip be produced, but also as a result of its treatment the strip remains permanently flat.